The present invention relates to a cascade-comparator analog-to-digital (A/D) converter.
An A/D converter is known in which a first-stage comparator A/D converter for providing the most significant bits of a digital output and a second-stage comparator A/D converter for providing the least significant bits of the digital output are cascaded. In the first-stage converter first voltage comparators compares an analog input signal level to be converted into digital form with a plurality of first reference voltage levels and a first decoder provides the most significant bits in response to the comparison result. The most significant bits are converted by a digital-to-analog (D/A) converter into an analog voltage, and the difference between the converted analog voltage and the analog input signal is obtained. In the second stage converter, the analog voltage difference is compared by second voltage comparators with a plurality of second reference voltage levels, and a second decoder produces the least significant bits on the basis of the comparison result. The step size of the second reference voltage levels is set to 1/2.sup.n (n represents the number of the least significant bits) of the step size of the first reference voltage levels.
Such a cascade-comparator A/D converter can perform in principle an A/D conversion with high accuracy and at high speed. When the conversion accuracy of the local D/A converter for converting the digital output of the first stage converter into an analog voltage is, however, insufficient, an error feasily occurs in the least significant bits. In other words, it was difficult to perform the A/D conversion in high accuracy by the conventional cascade-comparator A/D converter without providing a high accuracy D/A converter. The provision of the local D/A converter complicates the construction of the A/D converter.